This invention relates generally to optical networks. More particularly, the present invention relates to stabilizing light output of a laser device for producing light energy within an optical network.
Conventional lasers typically output unstabilized monochromatic light energy that has multiple spectral modes and that has more than one predefined wavelength region. The unstable light energy can be attributed to phenomena such as multiple longitudinal modes within the gain region of the lasing cavity, mode-hopping, drift, instability, and spontaneous emissions. These phenomena can be a dependent on the mechanics or physical configuration of a laser. Other phenomena which may negatively influence the light output of the conventional laser include the following: indeterminate back reflections of light into a laser cavity of the primary laser wavelength; unwanted or interfering light propagating at non-laser wavelengths entering into a lasing cavity; temperature instability; and manufacturing variations of the laser and associated optical components.
Numerous conventional architectures have been manufactured to spectrally control a laser device""s output. However, these conventional laser device architectures typically require large or bulky and optically inefficient components that may be subject to environmental elements. Some conventional laser device architectures require moving parts that can produce highly undesirable mechanical motions. Additionally, conventional laser device architectures that attempt to stabilize laser light output of the a laser device are usually permanently attached to the laser device. The conventional laser device architecture is simply not designed to be changeable or easily replaced.
As noted above, undesirable light at non-laser wavelengths can enter a laser cavity and significantly and negatively affect the output of the laser device. In other words, unwanted light at non-laser wavelength entering a laser cavity can substantially degrade the spectral purity of a laser device""s output.
Accordingly, a need in the art exists for a method and apparatus that can stabilize laser light output in addition to blocking any back reflections of light that may try to enter into a laser cavity. Another need exists in the art for a method and apparatus for stabilizing light output of a laser device. More specifically, there is a need in the art for an optical feedback assembly that can be utilized with conventional or existing laser devices. An additional need in the art exists for a method, and apparatus for stabilizing laser output that is readily detachable or replaceable as a field-configurable unit. There is also a need in the art for a method and apparatus that can stabilize laser light output where an optical configuration providing such a function has a unitary structure.
The present invention can solve the problems of conventional laser devices by providing an optical feedback assembly that comprises a filter and a partial reflecting device that can stabilize and center the optical output of a laser on a specific wavelength region. The partial reflecting device, such as a mirror, can be placed adjacent to the laser while the filter can be disposed between the partial reflecting device and the laser. Because of its relative location to the lasing cavity of a laser, the optical feedback assembly may be characterized as form of xe2x80x9cexternal cavity feedbackxe2x80x9d for the laser. The optical feedback assembly is well suited for optical networks in general and for dense wavelength division multiplexing applications.
The filter of an optical feedback assembly can be designed to pass only a predetermined wavelength region of light energy. In one aspect of the invention, the filter can be a thin film interference filter. After light energy having the predetermined wavelength region passes through the filter, it can be reflected by the partial reflecting device back into a lasing cavity of a laser so that the laser can xe2x80x9clock onxe2x80x9d to the predetermined wavelength region. In other words, the light energy having the predetermined wavelength region reflected by the partial reflecting device into the lasing cavity can permit the laser to output light energy centered on the predetermined wavelength region.
The optical feedback assembly can be designed to work with conventional lasers, such as off-the-shelf semiconductor lasers. The optical feedback assembly can enhance the output of such conventional lasers. For example, conventional lasers, such as Fabry-Perot diode lasers, typically produce light energy that is monochromatic and exhibits multiple modes at numerous wavelength regions. When coupled to the inventive optical feedback assembly, the output of a conventional laser can be adjusted such that the laser produces highly monochromatic light energy at a predetermined wavelength region. The optical feedback assembly can achieve these results at very low cost and with little or no modification to the conventional laser itself since the optical feedback assembly can be readily attached to the laser.
The optical feedback assembly can be formed into an integrated optics package. That is, the filter and the partial reflecting device can form a single or unitary construction. Each integrated optics package can be easily disposed within an optical waveguide or at an end portion of an optical waveguide to form field configurable stubs. In other words, each optical feedback assembly can be integrally formed within or attached to small optical waveguides that can be easily coupled to conventional lasers. Such construction permits rapid and cost efficient replacement or switching of parts.
For example, in order to change the operating wavelength region of a conventional laser, all that may be needed is a stub having an optical feedback assembly designed to filter and reflect light energy at a different wavelength region. On the other hand, conventional laser technology requires rather complex optics and electronics and physical manipulation of the lasing cavity to achieve different wavelength regions of laser light output. With the present invention, however, an existing stub filtering and reflecting light energy at a first wavelength region can be replaced by a stub that filters and reflects light energy at a second wavelength region different from the first wavelength region.
According to another aspect of the present invention, the optical feedback assembly may further comprise an anti-reflective coating that can be disposed on a front or output facet of a lasing cavity of a laser. Conventional lasing cavities of lasers typically have reflective characteristics for output facets in order to permit lasing. With the present invention, an anti-reflective coating disposed on the output facet of the lasing material permits a laser to enhance its output by more readily xe2x80x9clocking onxe2x80x9d to the light energy of a predetermined wavelength region reflected by the partial mirror.
For another aspect of the present invention, the optical feedback assembly may comprise a grating instead of a thin film interference filter. More specifically, the optical feedback assembly may comprise a Bragg grating in combination with an optical waveguide. The Bragg grating may be disposed within or adjacent to an optical waveguide. Because light energy can be waveguided within a Fiber-Bragg grating, this optical feedback assembly can produce very efficient laser light output at predefined wavelength region regions.
In another aspect of the present invention, the optical feedback assembly may further comprise a guard-band filter disposed outside of or on an opposite side of a Bragg grating. The guard-band filter can prevent light energy of undesired wavelength region regions from passing through the Bragg grating into the lasing material of the laser. In other words, the guard band filter can prevent signals generated by a network from propagating back through the Bragg grating into the lasing cavity.